CRA provides an example of the election to claim medical expenses (that are not a prepayment for a future year’s services) on a lagged basis

After noting that fees (e.g., to a dentist) will not be recognized as medical expenses until they are incurred (i.e., the related services are performed (so that, for example, fees prepaid in 2015 for services to be performed in 2016 would not qualify as medical expenses in 2015), CRA went on to confirm (somewhat contrary to an earlier “confusing” 2005 technical interpretation) that for purposes of computing the medical expense tax credit ("MEC") in s. 118.2(1) for a year, an individual may choose any 12-month period ending in the year, but that the medical expenses must have been paid during the period so chosen. For example, if dental fees were paid on December 31, 2015, they would not be eligible for the METC computation for the 2016 taxation year if the individual had selected the calendar year, but such fees could be included in the computation of the METC in the 2016 return if the selected period included December 31, 2015.

Neal Armstrong. Summaries of 6 December 2016 External T.I. 2015-0589041E5 Tr under s. 118.2(1) and s. 118.2(2)(a).